Halftone techniques have long been used to produce color images. The art of digital halftoning involves conversion of a contone image, in which image elements each have a color tone value, to a binary, or halftone representation, in which image elements are either “on” or “off.” Color tone values become binary dot patterns that are intended to be averaged by the human eye and viewed as the desired color tone.
In digital imaging, halftoning usually involves using a threshold matrix or “mask” to simulate the classical optical screen, which is tiled to cover the whole page, or a dynamic error-diffusion procedure that averages the errors caused by the device limitations throughout the page.
The threshold matrix is an array of threshold values that spatially correspond to the addressable points on the output medium. At each location an input value is compared to a threshold to make the decision whether to print a dot or not. The matrix can be used on a large image by applying it periodically. Typically, this “screen” produces halftone dots that are arranged along parallel lines in two directions, i.e. at the vertices of a parallelogram tiling the plane with a given frequency and at a given angle.
Printing presses use plates to print ink onto paper and other media. One method used for creating plates is to expose photosensitive film with the matter to be printed. When the film is developed, the matter imaged on the film may be imaged onto a photosensitive plate, sometimes referred to as “burning” a plate. After processing, the plate can be used on a press to print the matter onto a medium. Part of the plate, usually the part defining the image to be printed, retains ink, while the other part of the plate does not. When the plate is introduced to ink and then to paper or other medium, the image is printed on the medium.
In a black and white printing job, there is usually one plate that is used to print black ink. In a color printing job, a different plate may be used for each color ink. A color job may use three colors of ink, usually cyan, magenta, and yellow, which in combination can be used to make other colors. A plate is usually produced for each color ink. Often, in addition to cyan, magenta, and yellow inks, black ink is also used. An additional plate is then required to print the black ink. Occasionally, one or more colors will be printed separately as well, referred to as a “spot color.” That color will also have its own plate.
Electronic prepress systems have used an imagesetter to receive raster data associated with a plate and to image the raster data onto photosensitive film. In this context, a raster may specify an image by pixels in columns and rows. The film is then used to create a plate. The imagesetter exposes the photosensitive film pixel by pixel. One way that imagesetters image the raster data is to scan a laser across and down a piece of film. Electronics control the laser to expose, or refrain from exposing, each pixel in the raster data. The imagesetter images the pixels on the film in a manner that is precise and repeatable. Recently, platesetters also have been used to create plates directly from raster data without the use of intermediate film. Imagesetters, platesetters and like print engines, including proofers, are also referred to generally as output devices. In this context, imagesetters, platesetters and output devices used to image on material used to make a plate, for example on film and plate, are referred to as final output devices.
Print engines typically have been served by a dedicated raster image processor (“RIP”) connected between the print engine and a “front end” computer running imaging application software. An example of such a front end computer is an Apple Macintosh™ running Quark Express™ imaging software. Other examples include, but are not limited to, Adobe Pagemaker™ and Luminous PressWise™ software. In a typical configuration, the Macintosh™ front end computer is connected to an Agfa Atlas™ RIP, which communicates with an Agfa Accuset™ imagesetter. The RIP interprets the graphic information transmitted to it by the front end, and converts the graphic information into raster data that can be imaged by the print engine. The raster data produced by the RIP typically depends on the capabilities of the imagesetter, such as the imaging resolution and the media type and media size loaded into the imagesetter.
In particular, the conversion of the contone image to a high-resolution halftone image using, for example, periodic halftoning mentioned above, is typically performed by the RIP. When an image file coded as a page description language is received by the RIP, operations performed by the RIP, such as using fonts to lay out text and using color processing to create raster data for each color, results in one or more raster data bit maps. The raster data produced by the RIP is usually binary, meaning that each pixel is either on or off, as distinguished from a contone raster data set, which includes at least one color level for each pixel. The raster data for each of the colors in a color image are referred to as color “separations.” Each separation is transferred from the RIP to the output device over a high speed interface.
High-resolution color images are represented in separations. Each separation is imaged separately to the imagesetter or platesetter. The separations imaged by the output device are used to make printing plates (in the case of imagesetters) or are the printing plates themselves (in the case of platesetters).
When an output device images on film, it is possible to inspect the film before creating a plate. If the image on the film contains an error that may be observed by inspecting the film, such as misaligned, misplaced, or incorrectly included or excluded graphics, text, or colors, the error can be corrected, and a new film imaged. The corrected film may then be used to create the plate. When the output device images directly onto a plate, there is no intermediate film step. It is, therefore, not possible to observe the results of RIP processing until the plate is made.
Thus, when an output device outputs directly onto a plate, and often when imaging onto film, prior to generating the final image, it is helpful and, indeed, desirable to first produce a “preview” version of the image. Such simulations of the final, printed product, are known as “proofs.” Proofs are typically used for customer printing contracts, for example, to secure customer's approval prior to creating the final product. Accordingly, proofs need to accurately represent the content, color separation, accuracy of imposition and other parameters of the image.
The quality of the preview image or proof, however, is limited by the quality of preview output devices. Preview output devices are commercially available, and are sometimes referred to as proof printers or proofers. These output devices generally accept page description language image files as input and produce images that integrate one or more separations onto a single sheet of paper or film. The proofer output, i.e. the proof, may resemble the final image output that will be produced by the press. The proof image may be used to verify text, graphics, and color layout. The effectiveness of the proof image is determined by the extent that the proof image resembles the final output.
One reason that a proof image may not resemble the final output is that the page description language interpreter (RIP) used by a proofer may interpret the same page description language differently than the RIP that provides raster data to the final output device. When the print job containing images coded in a page description language is received from the front-end by the RIP, the images are typically first RIP-processed for the proofer. Images generated for output to the proofer are typically contone images (meaning that each pixel has some color value) rather than binary separations. If the proof images are satisfactory, the images are then processed by the RIP again to a target high resolution output device in a pixel format. Thus, the RIP output for the proofer will typically be different than the RIP output generated for an imagesetter or platesetter. In this case, there may be significant differences between a final image and the proof image. For example, on the proof image there may be differences in the overlay of image elements resulting in text that may appear to be missing or covered by graphics, while this artifact may not be visible in the final image. As another example, text may appear to be properly placed on the proof image, but be incorrectly placed in the final image. Furthermore, this process is inefficient because multiple RIPs, multiple RIP setups, and additional RIP time and image quality assurance are required. Also, page images can also be received by the electronic prepress system in a pixel format such as, for example, TIFF. Jobs received in TIFF format have already been processed by the RIP at high resolutions.
Accordingly, it is desirable to generate proofs from the same set of high-resolution image data that is subsequently used to produce a final image, for example, RIP process the job for the high resolution output device, and then reuse the same image data to create the color proof without having to re-process by performing a second RIP operation.
High-resolution image data contains billions of 0's and 1's representing text characters, rules, filled shapes and halftone images. To generate a proof, it is necessary to restore the original color intensities (or ink densities) and obtain a contone equivalent of the halftoned image. This process is often referred to as descreening because it removes the screen that was applied while halftoning. Thus, in order to generate lower resolution continuous tone image, the billions of high-resolution image pixels must be sampled, analyzed and converted to lower resolution image pixels. The original object coded in the high-resolution pixels, however, may not be known. For example, it may be solid or a screened shade of gray. The halftoning process necessarily loses some image information in the conversion of the original continuous tone image to a halftone image. The reconversion of a halftone image to a continuous tone image, therefore, is essentially an estimation process because the halftoning process cannot be reversed exactly to reproduce a continuous tone image identical to the original image. Typical problems encountered while converting high resolution data to create lower resolution images include excessive contrast between gray levels, interference patterns, and/or resulting lower resolution images being blurred or too soft.
Descreening solutions known in the art are typically expensive in terms of processing time and computing resources. For example, known descreening methods require either multiple computational steps over the data, such as U.S. Pat. Nos. 5,343,309; 6,101,285; and 6,222,641, analysis of the source halftone image to determine filters or sampling parameters, such as U.S. Pat. Nos. 4,630,125; 5,027,078; and 6,172,769), or repetitive algorithms, such as U.S. Pat. No. 5,339,170.
U.S. Pat. No. 6,252,676 B1 to Azima et al. (“Azima '676”) discloses a system for displaying an image on an output device includes a raster image processor for processing the description of the image in the page description language thereby creating a raster for each color separation associated with the image. The system of Azima '676 also includes a preproofer for selecting a subset of the raster data sets for proofing, and for descreening, resizing and combining each of the subset of raster data sets for proofing to create a page description language file including the descreened, resized, and combined each of the selected subset of the raster data sets for proofing, as well as a proofer for imaging the resulting page description language file. The descreening can be accomplished by averaging the pixels in the raster. The number of pixels to be averaged is determined by dividing the RIP-processed (input) raster resolution by the proof (output) raster resolution. For example, to convert a source image at 2400 dpi to a 600 dpi proof, the system such as that of Azima '676 can examine 4×4 pixel squares (because 2400/600=4) and map the number of pixels in the source image to a tone value so that each pixel of the 4×4 area is used for one output pixel of the proof image. While this method simultaneously accomplishes descreening and resizing and is particularly simple to compute, in the areas containing halftoned objects of variable tone value it may produce undesirable interference patterns and excessive contrast between gray levels.